tree-optimization/113727 - bogus SRA with BIT_FIELD_REF

tree-optimization/113727 - bogus SRA with BIT_FIELD_REF

When SRA analyzes BIT_FIELD_REFs it handles writes and not byte
aligned reads differently from byte aligned reads.  Instead of
trying to create replacements for the loaded portion the former
cases try to replace the base object while keeping the wrapping
BIT_FIELD_REFs.  This breaks when we have both kinds operating
on the same base object if there's no appearant overlap conflict
as the conflict that then nevertheless exists isn't handled with.
The fix is to enforce what I think is part of the design handling
the former case - that only the full base object gets replaced
and no further sub-objects are created within as otherwise
keeping the wrapping BIT_FIELD_REF cannot work.  The patch
enforces this within analyze_access_subtree.

	PR tree-optimization/113727
	* tree-sra.cc (analyze_access_subtree): Do not allow
	replacements in subtrees when grp_partial_lhs.

	* gcc.dg/torture/pr113727.c: New testcase.